The principle of self-mixing interference has led to applications of self-mixing interferometers. The theory of self-mixing interference is as follows. When a portion of a laser beam is reflected by an external target back to a laser cavity, the reflected laser beam can have interference effect with the original laser beam inside the laser cavity. The laser output power can be modulated due to the interference effect. Such modulation is called self-mixing interference since the reflected laser beam is simply a reflection of the original laser beam. Because the reflected laser beam carries certain information of the external target, measurement of the laser output power can provide the information of the external target, such as velocity, surface and displacement, etc. Accordingly, such self-mixing interferometers have been used in different applications such as laser Doppler velocimeter, vibration detection, defect detection, and surface profile measurement, etc.
FIG. 1 is a front section view of a structure of a conventional laser self-mixing interferometer 10. Referring to FIG. 1, the laser self-mixing interferometer 10 includes a He—Ne laser discharge tube 12, a pair of laser cavity mirrors 14 and 16. The laser cavity mirrors 14 and 16 are attached to the laser discharge tube 12. Typically, a He—Ne laser including the laser discharge tube 12 and the laser cavity mirrors 14 and 16 can provide a He—Ne laser beam. Longitudinally disposed along the He—Ne laser are a photoelectric detector 18 and an object 20. The laser cavity mirror 14 allows 0.1%˜0.3% of the laser beam to be transmitted to the photoelectric detector 18. The laser cavity mirror 16 (also called laser output mirror) allows 0.5%˜1.5% of the laser beam to be transmitted to the object 20. The object 20 can be any shape, but must have a reflecting surface 22 to reflect laser beam back to the laser discharge tube 12. When there is a longitudinal displacement 24 of the object 20, the reflected laser beam and the original laser beam can have an interference effect inside the laser discharge tube 12. The photoelectric detector 18 receives the laser beams from the laser cavity mirror 14 and transfers the energy of the laser beams to an electric signal for displacement measurement.
FIG. 2 is a diagram showing the relationship between the displacement 24 and a laser intensity 26 that is detected by the photoelectric detector 18. Both theory and experiments have indicated that the laser intensity 26 changes an intensity period 28 when the displacement 24 changes a displacement period 30, which is a half of wavelength of the laser beam. An intensity curve 32 is similar to the curve of interferential fringe, though the intensity curve 32 is not an exact sinusoidal curve. Current research on self-mixing interferometer is based on such theory, and may choose semiconductor or CO2 laser to provide laser beam.
However, there are a number of disadvantages to use such type of self-mixing interferometer. For example, such self-mixing interferometer can not decide the direction of the displacement 24 of the object 20 if the reflected laser beam is not extremely strong. In particular, the intensity curve 32 can not show whether the displacement 24 is directed to left or right. If the object 20 moves from left to right for a displacement period 30 and then from right to left for another displacement period 30 back to original position, FIG. 2 will give wrong result indicating two period displacement instead of correct result that is zero displacement. Although certain documents disclose that the displacement direction can be decided according to a triangle-like waveform under the situation that the reflected laser beam is particular strong, such method has failed for its unacceptable measurement errors.
Another disadvantage of the current self-mixing interferometer is that the current interferometer lacks subdividing ability. In particular, the current interferometer has low resolution and cannot distinguish a displacement shorter than a half of the displacement period 30.